Who Are the Children of Lehi?

In 3 Nephi 5:20-23, Mormon, who abridged the record of the Nephites, reveals two significant points. First, he announces that he is a “pure” descendent of Lehi. As was common in ancient (and not so ancient) times, Mormon is obviously citing his paternal lineage. His maternal heritage was not “a pure descendant of Lehi,” as Lehi’s sons, at least the older ones, married daughters of Ishmael. What is Mormon’s motivation for making this distinction if the children of Lehi are the sole occupants of the Americas? Clearly Mormon’s statement acknowledges that the Book of Mormon is a lineage record, concerned with one particular family line among many.